Compiling Specific Versions of Unison (File Sync Application)

Unison can be very fussy about which version of the application and which version of the ocaml library you are using on both the client and server side. It can sometimes be a real chore to get all of the different devices on the same version and working well with each other, especially in a mixed architecture/OS environment.

Determine Your Target Version

Consider the various OSs you hope to support and choose the one you feel like could be the hardest to target the correct version on. Linux, MacOS, and Windows typically have binaries available from unison's releases page in github that will match most configurations so it would be wise to choose an OS or architecture that is not readily available for download.

In this example I am going to install the version in FreeBSD's package manager:

pkg update
pkg install unison

After installation, audit which version was installed by running the following command:

unison -version

In my case it returned: unison version 2.51.3 (ocaml 4.05.0). These are versions I need to target for all my other installations.

Download Unison Binaries for Common Configurations

Going to unison's releases page in github, you will need to locate the version of unison (not ocaml) that you need to download. Once, you have located the release you need expand the Assets section and download the specific ocaml version for the OS and architecture you need and install them in to your respective systems.

Compile Unison Binaries for Uncommon Configurations

Hopefully, you only have one uncommon machine you need a binary for, but if you have two or more you will more than likely need to compile unison for the specific platform you need. I chose to compile unison on an ARM/Linux machine by doing the following.

Get the Correct Version of Unison

Check out the version of the source to match your target version. In this example I am looking for version 2.51.3 of unison:

git clone https://github.com/bcpierce00/unison.git
cd unison
git checkout v2.51.3

Switch to the Correct Version of opam/ocaml

Install opam from your package manager:

sudo apt install opam

Initialize your opam environment:

opam init

When prompted with the following two choices choose no for both:

Do you want opam to modify ~/.profile? [N/y/f]
(default is 'no', use 'f' to choose a different file)

A hook can be added to opam's init scripts to ensure that the shell remains in
sync with the opam environment when they are loaded. Set that
up? [y/N]

Switch your opam environment to the correct ocaml version you need by running the following command (be sure to switch out the version in the example with the version you need):

opam switch create 4.05.0

Update the current shell environment to use the version of opam that was selected:

eval $(opam env)

Compile Unison

Run make commands to build the application:

make clean
make

Check the version you have built:

./src/unison -version

If everything looks good, run the installer script (or just manually install the built binary)

make install